1. Field of the Invention
The present invention relates to an alkaline storage battery, a hydrogen-absorbing alloy used therefor, and a method for producing the same.
2. Description of Related Art
Nickel-metal hydride storage batteries practically made using hydrogen-absorbing alloys have the characteristics such as low environmental pollution and high energy density. The batteries are being widely used as electric sources of various cordless equipment and electronic equipment. Furthermore, from the points of their excellent capacity and reliability, these batteries are considered to be most promising for electric power sources of electric tools and electric vehicles which require charging and discharging at high-rate.
The charge and discharge reaction at a negative electrode comprising a hydrogen-absorbing alloy in nickel-metal hydride storage batteries is shown by the following formula (1). In the charging stage, the hydrogen-absorbing alloy negative electrode electrochemically takes hydrogen atom into the alloy from water. In the discharging stage, the hydrogen absorbed in the alloy is electrochemically oxidized to return to water. 
As materials of negative electrodes for nickel-metal hydride storage batteries, generally used are MmNi5 (Mm denotes a mixture of rare earth elements) alloys having a crystal structure of CaCu5 type in which a part of Ni is replaced with a metal such as Co, Mn or Al.
In such hydrogen-absorbing alloy in the form of powders, an oxide or hydroxide is formed on the surface owing to spontaneous oxidation. The specific surface area of the alloy powders is small and the initial charge and discharge capacity as an electrode are low. Thus activation is needed. Furthermore, since hydroxides of rare earth elements are insulating, when they precipitate on the alloy surface, electric conductivity lowers and charge and discharge characteristics at high-rate become insufficient.
Moreover, the elements such as Mn and Al readily dissolve into an alkaline electrolyte and precipitate as an oxide or hydroxide. The dissolution and precipitation of these elements cause reduction in capacity of the hydrogen-absorbing alloy, increase of internal pressure in charging and deterioration of charge and discharge cycle life.
For enhancing activation of hydrogen-absorbing alloys, JP-A-61-285658 discloses a method of immersing hydrogen-absorbing alloy in an alkaline aqueous solution, and JP-A-3-152868 discloses a method of treating hydrogen-absorbing alloy with an acidic aqueous solution and then with an alkaline aqueous solution.
According to the disclosure of the above patent publications, elements on the surface of hydrogen-absorbing alloy which readily dissolve in an alkaline aqueous solution are removed by the alkali treatment. The removal of the elements results in formation of an active nickel-rich layer in the surface portion to improve the initial activation characteristics. However, since the alkali treatment results in formation of insulating hydroxides of low solubility (e.g., hydroxides of rare earth elements) on the surface of hydrogen-absorbing alloy, contact resistance between particles increases and electrical conductivity decreases. Furthermore, if the surface is covered with the hydroxide layer, the nickel-rich layer present under the hydroxide layer cannot sufficiently exhibit catalytic action. As a result, the initial activity and charge and discharge characteristics at a large current are insufficient.
In order to remove the hydroxides to enhance the high-rate charge and discharge characteristics, JP-A-5-225975 discloses a method of treating with a strongly acidic aqueous solution such as hydrochloric acid, and JP-A-9-171821 discloses a method of treating with an alkaline aqueous solution and then with an acidic aqueous solution. These methods remove the hydroxide film formed on the alloy surface by an acid treatment, and form a nickel-rich layer on the alloy surface to improve activity of hydrogen-absorbing alloy and high-rate charge and discharge characteristics.
However, according to the treatments of the above conventional techniques, hydrogen is produced by the alkali treatment as shown in the formula (2) and by the acid treatment as shown in the formula (3). A part of the produced hydrogen is released as a gas as shown in the formula (4), but most of the hydrogen is absorbed in the hydrogen-absorbing alloy as shown in the formula (5). When hydrogen is absorbed in the alloy, this reacts with oxygen in the air at the step of being exposed to the air for drying or the like as shown in the formula (6) to generate heat (Q). The nickel portion on the surface of the hydrogen-absorbing alloy is oxidized owing to the generation of heat, resulting in reduction of catalytic effect.
M+H2Oxe2x86x92MOH+Hxe2x80x83xe2x80x83(2) 
M+H+xe2x86x92M++Hxe2x80x83xe2x80x83(3) 
2Hxe2x86x92H2xe2x80x83xe2x80x83(4) 
M+Hxe2x86x92MHxe2x80x83xe2x80x83(5) 
4MH+O2xe2x86x924M+2H2O+Qxe2x80x83xe2x80x83(6) 
(M: Hydrogen-absorbing alloy)
As a result, the initial activity and the high-rate charge and discharge characteristics decrease. Moreover, hydrogen produced at the high-rate charging is difficult to be rapidly absorbed into the hydrogen-absorbing alloy to cause increase of internal pressure and deterioration of battery cycle life.
Thus, sufficient high-rate discharge characteristics at the initial charge and discharge cycle cannot be obtained only by the removal of the oxide layer or hydroxide layer on the surface of the hydrogen-absorbing alloy or the formation of the nickel-rich layer on the surface of alloy powders according to the above conventional techniques, and this is not sufficient in the use for electric vehicles or electric tools which are demanded to have a high output.
The main object of the present invention is to provide an alkaline storage battery which is excellent in internal pressure characteristics, charge and discharge cycle life and high-rate discharge characteristics.
For attaining the above object, the present invention provides a hydrogen-absorbing alloy for batteries in the form of powder comprising at least one rare earth element, nickel and at least one transition metal in which nickel in metallic state is exposed at the surface portion of the alloy, pores are positioned between said nickel and nickel, and a nickel-rich layer is present on the alloy surface which contacts with the pores.
The above alloy is produced by a method which comprises a first step of grinding a hydrogen-absorbing alloy comprising at least one rare earth element, nickel and at least one transition metal to prepare alloy powders, a second step of treating the alloy powder in an alkaline aqueous solution, a third step of treating the alloy powders in an acidic aqueous solution, and a fourth step of dehydrogenation treatment to remove hydrogen absorbed in the alloy powders in the presence of acetate ion in an aqueous solution. More preferably, the method comprises a fifth step of adding an alkali to the aqueous solution after the fourth step to modify with OHxe2x88x92 group the surface of the alloy powders and the surface of alloy contacting with the pores.